Field of the Invention
The invention relates to a method for protecting a heated temperature-dependent sensor resistor against overheating, which includes connecting the sensor resistor in a first bridge branch of a bridge circuit for measuring a drawn-in air mass in an intake manifold of an internal combustion engine; connecting a resistor as an air temperature detector in a second bridge branch of the bridge circuit; and controlling a bridge current through a first circuit for keeping a prescribed bridge diagonal voltage constant and for serving as a measure of the drawn-in air mass.
The invention also relates to circuit configuration for measuring a drawn-in air mass in an intake manifold of an internal combustion engine, including a heated temperature-dependent sensor resistor connected in a first bridge branch of a bridge circuit; a resistor disposed in a second bridge branch as an air temperature detector; a first differential amplifier and a circuit controlling a bridge current as a measure of the drawn-in air mass for keeping a prescribed bridge diagonal voltage constant; a third bridge branch connected in parallel with the first bridge branch as a voltage divider; and a second differential amplifier having inputs connected to center taps of the first and third bridge branches, as well as a further circuit.
Such a method is disclosed in German Published, Non-Prosecuted Patent Application DE 32 07 394 A1, corresponding to U.S. Pat. No. 4,462,251. There, the sensor resistor is protected against overheating, although only during burn-off operations for removing deposits.
An appropriate circuit configuration is disclosed in German Published, Non-Prosecuted Patent Application DE 30 22 685 A1, corresponding to U.S. Pat. No. 4,420,971, where a precise setting of a specific wire temperature is undertaken, likewise by burning off an air mass meter, with a circuit configuration having three bridge branches and two differential amplifiers, in which configuration a maximum prescribed sensor temperature of the sensor resistor is not exceeded.
The construction and the mode of operation of thermal air mass meters is generally known, for example from German Patent 20 42 983. That device is described in greater detail below with regard to FIG. 3.
Conventional air mass meters normally have a certain inertia, so that backfires in the intake manifold do not have a damaging influence on the sensor resistor. However, because of increased technical requirements, quickly reacting air mass meters are being used more and more. If backfires occur in the intake manifold, the differential amplifier attempts to correct the bridge which has been detuned thereby on the basis of heating of the temperature detector, and that can lead to overheating and thus to damage to the sensor resistor.